However, the strategy of designing viruses that can leverage the large and rapidly growing armamentarium of animal lines that express exogenous recombinases only in defined cell types (driver lines, which can fully capitalize on enormous native promoter/enhancer regions rather than the small fragments which fit Erastin nmr into viruses) offers an expanded range of opsin targeting strategies (Figure 2B; see Table 3 for driver lines used in optogenetic studies). New driver lines
are continually added to the available repertoire by groups such as GENSAT and the Allen Institute for Brain Science. Successfully utilizing a recombinase driver line requires efficient packaging of the genetic material to be expressed into a recombinase-dependent system conferring the two properties of (1) very low leak (background) of opsin expression in non-recombinase-expressing cells, and (2) very high recombinase-induced opsin expression—all within the viral backbone. Several potential different recombinase-dependent viral vector designs have emerged (Kuhlman and Huang, 2008, Zhang, 2008, Atasoy et al., 2008 and Sohal et al., 2009), and a Cre recombinase-dependent double-floxed inverted opsin gene in AAV under the EF1α promoter (Zhang, 2008 and Sohal et al., 2009) or the CAG promoter
(Atasoy et al., 2008) was ultimately found to provide a suitable combination of strength and specificity to enable behaviorally significant optogenetic gain or loss of function within the constraints of the freely moving mammal system see more (Tsai et al., 2009 and Aponte et al., 2011). Not only is this strategy versatile in the sense that it can be applied at will to the large and growing pool of Cre driver lines (e.g., Gong et al., 2007), soon to include rat as well as mouse lines, but this approach is also by design expandable along new dimensions that enable combinatorial experiments (Figure 2). First, other recombinases such as Flp or Dre may be used to construct orthogonal driver lines that can be crossed with Cre driver lines while the same low-leak, high-potency recombinase-dependent AAV design
is theoretically adaptable for these other recombinases as well. Second, promoter fragments may be ADP ribosylation factor used at the same time in place of the EF1α promoter in the recombinase-dependent viruses, thereby implementing intersecting promoter and recombinase-dependent specificity. Third, while generation of recombinase-dependent opsin mouse lines for simply crossing with Cre driver lines is a viable approach (Madisen et al., 2010a, Madisen et al., 2010b, Kätzel et al., 2011 and Zariwala et al., 2011), resulting opsin expression levels may be weaker than with high-copy-number recombinase-dependent viruses, and more importantly the viral approach provides a unique advantage of intersecting genetic and anatomic specificity. To illustrate this point, consider that for most Cre driver lines, specificity exists only at particular points in space and time.